The invention relates generally to a method of copying colored originals such as, for example, the frames of exposed and developed strips of color film.
More particularly, the invention relates to a method of determining the amount of light in each primary color to which a colored original is to be exposed during copying.
In a known method of determining the amount of light in each primary color to which a colored original is to be exposed during copying, each of a series of regions or points of the original is scanned in the three primary colors, e.g. to measure the transparency of the original to light of each color. The measurements obtained from each region are compared with reference values in order to determine whether the respective regions contain color dominants. The amount of light in each primary color to which the original is to be exposed during copying is then calculated disregarding the regions with color dominants altogether or giving less weight to such regions.
The so-called neutral gray compensation principle employed in color copying is capable of compensating for color tones which are inherent in the film or paper constituting the original or negative. However, the use of the neutral gray compensation principle for automatic exposure control leads to difficulties when the motif of the original or negative contains intensively colored areas, that is, so-called color dominants. If the neutral gray compensation principle is uniformly applied to such an original, a color shift occurs in the copies rendering the copies unusable. This gives rise to the problem of distinguishing between color tones and color dominants and applying the so-called neutral gray correction to color tones only.
Neutral gray correction is performed by selecting the amount of copying light in each primary color so that the copy has a neutral gray color composition on average. This may be accomplished using a photoelectric measuring device. The correct amount of light in each primary color may be obtained by regulating the exposure time in the respective color or by regulating the intensity of the light in the respective color.
The West German Auslegeschrift No. 25 35 034 discloses a photographic color copier having a photoelectrically regulated exposure control unit. Various regions of an original are scanned in each primary color and ratios of the measured values for different colors are formed for each region. When a clear color imbalance exists for a particular region, special measures are employed for the respective region, e.g. assigning only partial weight to such region in calculating a gray value or constant value for neutral gray correction. These measures are intended to reduce or eliminate the effects of dominants during determination of the amounts of copying light required for neutral gray compensation.
In the copier of the West German Auslegeschrift, the densities of the individual colors in each region are compared in order to determine whether one of the three primary colors predominates relative to the others. Ratios of blue density/green density, green density/red density and/or red density/blue density are formed and compared with reference values. Since the densities of the masks used in the production of film vary from one manufacturer to another, the reference values for different types of film must be different if reliable results are to be obtained. In determining the amounts of copying light required for an original, it is therefore necessary to provide data relating to the manufacturer of the film. This information can only be fed into the copier manually, particularly when a copy of an original is back ordered and the original constitutes a frame of one of a series of so-called multiple frame strips, that is, strips which respectively contain four or five originals. Manual input of information relating to the manufacturer increases labor costs. In another known method of determining the amount of copying light in each primary color, a so-called reference density in each primary color is employed as a comparison to determine whether inequalities in the densities of the individual colors indicate the presence of a color dominant. The reference densities for a specific type of film are derived from the results of measurements made on a large number of originals produced by the same manufacturer. Here, also, it is necessary to provide information on the type of film constituting an original to be copied in order to perform a copying operation. Furthermore, this method involves the expense of determining reference densities in the three primary colors from a large number of originals for each type of film to be copied.
An additional drawback of the methods outlined above stems from the fact that the film type indicated at the edge of a film identifies the photometric properties of the film under proper processing conditions. However, deviations from the normal processing parameters for a film may occur if, for instance, the film is stored for too long a period or at too high a temperature or a latent image on the film undergoes changes because of excessive storage time, particularly at elevated temperatures. Departures from the expected photometric properties of the film always result in color shifts.